An automatic parking brake (APB) is known, for example, from DE 10 2011 005 842 A1. Said automatic parking brake comprises a control unit which interacts with two actuators which are in each case arranged on the two rear wheels of a motor vehicle. An actuator of this kind is generally a (DC) electric motor which can shift a brake piston by means of a gear mechanism and a spindle drive in order to exert a clamping force on a brake disk. Owing to this mechanical locking, the automatic parking brake replaces the parking brake function, which is usually realized by means of Bowden cables, of a conventional handbrake. The control unit for the automatic parking brake is located either in the interior of the vehicle or is integrated into an existing control device, such as the electronic stability control device ESC for example. In order to actuate the two actuators, the control unit is connected to said actuators via lines, in particular via copper lines, which are laid in the vehicle. Laying the lines from the vehicle body, in particular from the wheel arch, to the actuators which are arranged on the rear axles presents a particular challenge when laying the cables since, at this point, the lines are freely accessible and subject to considerable stresses due to environmental influences, such as being struck by stones, moisture, being bitten by rodents etc. Stresses of this kind can damage the lines and this can lead to short circuits. These short circuits generate an overcurrent, which can have a negative impact on the functioning of the parking brake. Therefore, it is of critical importance for reliable operation of the automatic parking brake that a short circuit which may occur on said lines is identified and indicated to the driver.
In conventional systems, short-circuit identification in the parking brake system is realized by monitoring the current carried in the lines, so that, in the case of a short-circuited actuator, the overcurrent is detected and the actuator is consequently switched off. However, in order to be able to detect an overcurrent at all, detection of this kind takes place only in the actuation state of the actuator, that is to say only when the parking brake is applied. Short-circuit identification between the two feed lines which connect the actuator to the control unit has, to date, not been possible when the actuator is not in the actuation state. In particular, actuation of the actuator during driving of a vehicle for checking the connecting lines for short circuits cannot be carried out since the actuator, which is associated with a rear wheel, would move during actuation and consequently would initiate a parking brake function during driving of the vehicle.